The present invention relates to an optical multiplexer/demultiplexer and, more particularly, to such an arrangement which utilizes focusing Bragg reflectors.
In many optical communication systems, there exists a need to perform multiplexing/demultiplexing of optical signals having different wavelengths, known as wavelength division multiplexing (WDM). Preferably, these operations are provided by some sort of monolithic, integrated optical structure. Bragg reflectors, comprising a series of parallel grating lines, are often used in these devices as wavelength-selective filters.
One particular integrated optical multiplexing/demultiplexing scheme is disclosed in U.S. Pat. No. 4,740,951 issued to J. Lizet et al. on Apr. 26, 1988. Lizet et al. disclose an integrated structure formed on a silicon substrate including an input waveguide and collimating means, a plurality of n Bragg reflectors with an associated plurality of n focusing mirrors, and a plurality of n output waveguides. The n Bragg reflectors, constructed by etching layered dielectric films on a silicon substrate to form linear gratings, are arranged in cascade fashion where each functions to reflect a different one of the n possible wavelengths of transmission. The etched mirrors are utilized to focus the reflected wavelengths into the core region of the corresponding output waveguides. Although this arrangement is capable of achieving optical multiplexing/demultiplexing, the utilization of etched mirrors as focusing elements is considered to introduce unacceptable scattering and coupling losses into the system.
An alternative monolithic arrangement is disclosed in U.S. Pat. No. 4,746,186 issued to A. J. A. Nicia on May 24, 1988. Nicia discloses an integrated optical multiplexer/demultiplexer wherein the input and output fibers are attached to the same face of the device. A doubly periodic Bragg reflector, formed by etching dielectric films on a silicon substrate, is used to separate the various wavelengths and direct each signal to its associated output fiber. Since a single Bragg reflector is used to redirect all of the wavelengths into a number of different output channels, the reflector must have a relatively large bandwidth sufficient to cover all of the channels. Such a broadband reflector tends to be polarization dependent and lossy, due to scattering and coupling losses.
Thus, a need remains in the prior art for a monolithic optical multiplexer/demultiplexer arrangement which is relatively compact and simple to manufacture, providing reproducible results, yet capable of low insertion loss and polarization independent operation.